556 PATHOGENIC MICROORGANISMS 



special methods, be shown to possess a gelatinous capsule. It does 

 not possess fiagella and does not form spores. 



The plague bacillus is easily stained with all the usual anilin 

 dyes. Diluted aqueous fuchsin and methylene-blue are most frequently 

 employed. With these stains the characteristically deeper staining 

 of the polar portions of the bacillus is usually easy to demonstrate. 

 Special polar stains have been devised by various observers. Most of 

 these depend upon avoidance of the usual heat fixation of the prepara- 

 tions, which, in some way, seems to interfere with good polar staining. 

 Fixation of the dried smears with absolute alcohol is, therefore, prefer- 

 able. The bacillus is decolorized by Gram's method. 



Pig. 119. — Bacillus pestis, Involution Forms. (After Zettnow.) 



Isolation and Cultivation. — The bacillus is easily isolated in pure 

 culture from the specific lesions of plague patients, during life or at 

 autopsy. It grows readily and luxuriantly upon the meat-infusion 

 media. The optimum temperature for its cultivation is about 30° C. 

 Below 20° C. and above 38° C, growth is sparse and delayed, though it 

 is not entirely inhibited until exposed to temperatures below 12° C, 

 or above 40° C. The most favorable reaction of culture media is neu- 

 trality or moderate alkalinity, though slight acidity does not prevent 

 development. 



On agar, growth appears within twenty-four hours as minute 

 colonies with a compact small center surrounded by a broad, irregularly 

 indented, granular margin. 



